In recent years, with an increase of sensitivity to global environment, polylactic acid receives attention as a carbon neutral material that is a substitute for general resins produced from the conventional petroleum resources. Polylactic acid is produced from a plant such as corn as a starting material and is a thermoplastic resin that is regarded as being of a low environmental load type from the standpoint of carbon neutral. Such polylactic acid is expected to be used as an environmentally gentle, plant-derived general resin for foams. Thus studies are being made on foams made of polylactic acid as a raw material. Among such foams, polylactic acid expanded beads molded articles can be obtained by in-mold molding in any desired shape without restriction, similar to conventional polystyrene expanded beads molded articles and polyolefin expanded beads molded articles. Such molded articles are, thus, promising in that they are likely to allow easy design of properties according to the aimed lightness in weight, cushioning property and heat insulating property.
As regards polylactic acid expanded beads and polylactic acid expanded beads molded articles, inventions disclosed in Patent Documents 1 to 3 have been hitherto proposed.
JP-A-2000-136261 discloses expandable resin particles of an aliphatic polyester such as polylactic acid, in which a volatile blowing agent such as n-pentane has been impregnated in a temperature range in which the degree of crystallization is 0 to 20%. An expanded beads molded article of JP-A-2000-136261 obtained from polylactic acid is prepared by placing the expandable resin particles in a mold and heating them with hot wind to expand and, at the same time, fuse bond the beads together and, therefore, has problems that the density significantly varies with position thereof, fusion bonding between expanded particles and the dimensional stability thereof are not sufficient, and mechanical properties are not satisfactory.
JP-A-2004-83890 discloses expanded beads of a polylactic acid resin which contains at least 50% by mole of lactic acid units, wherein the expanded beads show such characteristics in heat flux scanning differential calorimetry that a difference between the endothermic calorific value and the exothermic calorific value is 0 J/g or more and less than 30 J/g and an endothermic calorific value is 15 J/g or more (indicating that the crystallization has not sufficiently proceeded). Though the polylactic acid resin expanded beads disclosed in JP-A-2004-83890 show an improvement in fusion bonding property between the beads and in secondary expandability, there is a room for further improvement in fusion bonding property in view of the fact that the fusion bonding between the beads is not sufficient when the molded article has a complicated shape and the fusion bonding between the beads in a center region of the molded article is not sufficient when the molded article has a large thickness.
JP-A-2009-62502 discloses polylactic acid resin expanded beads wherein an endothermic calorific value (Rendo) thereof after a heat treatment is within a specific range, an endothermic calorific value (Bendo:J/g) and an exothermic calorific value (Bexo:J/g) thereof before the heat treatment have a specific relationship, and an exothermic calorific value (Bs:J/g) of a surface region thereof before the heat treatment and an exothermic calorific value (Bc:J/g) of a center region thereof have a specific relationship. In the expanded beads, the crystallization has not fully proceeded as a whole and, further, the degree of crystallization in the surface region thereof is lower than that in the center region. Therefore, the expanded beads show excellent inter-bead fusion bonding property and permit production of molded articles having a large thickness or a complicated shape. The above expanded beads, however, require a control of the degree of crystallization thereof in order to satisfy the above-described specific relationship and to obtain improved inter-bead fusion bonding and have a problem in the production efficiency because of the necessity of precise temperature control. For example, whilst it is relatively easy to obtain resin particles having a very low degree of crystallization by rapidly cooling the resin particles at the time the resin particles are formed, expanded beads obtained from the resin particles have so poor reproducibility in their expansion ratio and thermal characteristics that an expanded beads molded article showing good fusion bonding between the beads is not obtained in a stable manner, unless the temperature and time are severely controlled during a step of impregnating the resin particles with a blowing agent and a step of heating and expanding the resin particles impregnated with blowing agent. Therefore, a further improvement in the production efficiency of expanded beads molded articles is demanded.
As described in the foregoing, the conventional polylactic acid resin expanded beads have a problem with respect to fusion bonding between the expanded beads at the time of in-mold molding. Thus, there is a room for further improvement in fusion bonding between the expanded beads, especially when an expanded beads molded article having a complicated shape or a large thickness is to be obtained. The objective problem of the present invention is to provide polylactic acid resin expanded beads showing excellent fusion bonding property in in-mold molding thereof.